The modern shoe, particularly an athletic shoe, is a combination of many elements which have specific functions, all of which must work together for the support and protection of the foot. Athletic shoes today are as varied in design and purpose as are the rules for the sports in which the shoes are worn. Tennis shoes, racquetball shoes, basketball shoes, running shoes, baseball shoes, football shoes, weightlifting shoes, walking shoes, etc. are all designed to be used in very specific, and very different, ways. They are also designed to provide a unique and specific combination of traction, support and protection to enhance performance. Not only are shoes designed for specific sports, they are also designed to meet the specific characteristics of the user. For example, shoes are designed differently for heavier persons than for lighter persons; differently for wide feet than for narrow feet; differently for high arches than for low arches, etc. Some shoes are designed to correct physical problems, such as over-pronation, while others include devices, such as ankle supports, to prevent physical problems from developing.
A shoe is divided into two general parts, an upper and a sole. The upper is designed to snuggly and comfortably enclose the foot, while the sole must provide traction, protection, and a durable wear surface. The considerable forces generated by running require that the sole of a running shoe provide enhanced protection and shock absorption for the foot and leg. It is also desirable to have enhanced protection and shock absorption for the foot and leg in all types of footwear. Accordingly, the sole of a running shoe typically includes several layers, including a resilient, shock absorbing or cushioning layer as a midsole and a ground contacting outer sole or outsole which provides both durability and traction. This is particularly true for training or jogging shoes designed to be used over long distances and over a long period of time. The sole also provides a broad, stable base to support the foot during ground contact.
The typical motion of the foot during running proceeds as follows. First, the heel strikes the ground, followed by the ball of the foot. As the heel leaves the ground, the foot rolls forward so that the toes make contact, and finally the entire foot leaves the ground to begin another cycle. During the time, that the foot is in contact with the ground, it typically is rolling from the outside or lateral side to the inside or medial side, a process called pronation. That is, normally, the outside of the heel strikes first and the toes on the inside of the foot leave the ground last. While the foot is air borne and preparing for another cycle the opposite process, called supination, occurs. Pronation, the inward roll of the foot in contact with the ground, although normal, can be a potential source of foot and leg injury, particularly if it is excessive. The use of soft cushioning materials in the midsole of running shoes, while providing protection against impact forces, can encourage instability of the sub-talar joint of the ankle, thereby contributing to the tendency for over-pronation. This instability has been cited as a contributor to "runners knee" and other athletic injuries.
Various methods for resisting excessive pronation or instability of the sub-talar joint have been proposed and incorporated into prior art athletic shoes as "stability" devices. In general, these devices have been fashioned by modifying conventional shoe components, such as the heel counter, and by modifying the midsole cushioning materials. For example, one technique incorporates a relatively stiff heel counter support over the heel counter, as shown in U.S. Pat. No. 4,288,929. A similar technique, wherein support is provided to a heel counter by a bead of material, is shown in U.S. Pat. No. 4,354,318. Another prior art technique to enhance motion control during foot impact is by building up the heel counter itself, such as shown in U.S. Pat. Nos. 4,255,877 and 4,287,675. Another technique is the use of higher density cushioning materials on the medial side of the shoe to resist pronation, such as shown in U.S. Pat. Nos. 4,364,188 and 4,364,189. The use of a less compressible or firmer fluid tight chamber in the medial heel area of a sole is disclosed in U.S. Pat. Nos. 4,297,797 and 4,445,283. Although these prior art techniques have exhibited a degree of success in controlling sub-talar joint motion and, hence, over-pronation, they have certain disadvantages. Generally, these techniques add to the weight and manufacturing expense of the shoes. Furthermore, the firmer, higher density foam midsole materials are subject to compression set and reduce the efficacy of the cushioning system.
The present invention was designed to take advantage of the lightweight cushioning capability of the materials used in current athletic shoes, while enhancing the stability of the shoes without incurring the above disadvantages of prior art "stability" devices.